It is well known that diesel engines which operate lean (i.e., defined to mean an air to fuel ratios above stoichiometric or lambda in excess of 1) and gasoline engines employing "lean burn" fueling strategies promote the formation of nitrogen oxides (NO.sub.x) in the products of combustion. One well known method for converting the NO.sub.x to harmless emissions, and the method to which this invention relates, is the introduction of a reductant, such as ammonia, to the exhaust gas stream containing NO.sub.x. The exhaust stream with the reductant, is contacted with a catalyst at elevated temperature to catalyze the reduction of nitrogen oxides with ammonia. This catalyzed reaction is referred to as the selective catalytic reduction ("SCR") of nitrogen oxides. The excess or residual ammonia present in the exhaust gas after SCR is released to the environment as NH.sub.3 slip or reacted with oxygen to form nitrogen, H.sub.2 O and possibly NO.sub.x across an oxidizing catalyst. Ammonia oxidation and SCR are competing reactions with the predominance of one over the other being controlled to some extent by the catalyst composition. See assignee's U.S. Pat. No. 5,516,497 to Speronello et al. and U.S. Pat. No. 5,024,981 to Speronello et al.
This arrangement is conceptually illustrated in FIG. 1A in which an engine 1, operated at lean conditions, produces exhaust gases into which a reductant 2, such as ammonia, is metered by valve 3. The stream of reductant and exhaust gases pass through a SCR catalyst 5 and then pass through an oxidizing catalyst (not shown) where hydrocarbons (HC), carbon monoxide (CO) and residual or excess ammonia are oxidized. This is the typical arrangement such as illustrated in U.S. Pat. No. 4,946,659 to Held et al., U.S. Pat. No. 5,189,876 to Hirota et al.; U.S. Pat. No. 5,224,346 to Berriman et al.; and U.S. Pat. No. 5,788,936 to Subramanian et al.
Emission regulations impose a limit on the quantity of specific emissions, including NO.sub.x, that a vehicle can emit during a specified drive cycle such as an FTP ("federal test procedure") in the United States or an MVEG ("mobile vehicle emission group") in Europe. These driving cycles specify an engine cold start. Catalytic converters, however, are most effective at elevated temperatures. The emissions produced by the engine while it warms to operating temperature can comprise a substantial portion of the total emissions produced during the entire drive cycle. To address this requirement emission systems employ several catalysts which are catalytically active at different temperature ranges. Insofar as SCR catalysts are concerned, a precious metal containing catalyst is catalytically active at lower temperatures compared to those formed with base metal oxides or zeolites.
A typical arrangement is illustrated in FIG. 1B which is similar to that shown in FIG. 1A except that SCR catalyst 5 now comprises a high temperature SCR catalyst 5A and a low temperature SCR catalyst 5B. As with FIG. 1A, an oxidizing catalyst downstream of SCR catalysts 5A and 5B can be provided. SCR catalysts 5A and 5B can be separate catalysts or separate beds within one catalyst and the oxidizing catalyst (not shown) could be configured as a separate end bed within one catalytic converter. While the placement of high and low temperature SCR catalysts can be varied, when an external reductant is added to the system, the high temperature SCR catalyst 5A is placed before the low temperature SCR catalyst. If the SCR positions were reversed, when the engine reaches normal operating temperature, the low temperature SCR's operating range or window is exceeded, and the ammonia will react with oxygen in the exhaust gas to produce N.sub.2 and H.sub.2 O or conceivably NO.sub.x. The SCR reaction with NO.sub.x to produce N.sub.2 and H.sub.2 O will not occur.
The configuration illustrated in FIG. 1B is specifically developed to minimize NO.sub.x emissions following cold start of the vehicle and represents a significant improvement over conventional systems illustrated in FIG. 1A. Nevertheless tests, as discussed below, have shown that the system is not responsive in a timely manner to reduce NO.sub.x during engine warmup following cold start. That is when the low temperature SCR catalyst operating temperature is reached during engine warm up, the low temperature SCR catalyst is not instantaneously reducing NO.sub.x. There is a time lag. Further, the system of FIG. 1B under certain load or driving conditions of the vehicle, and after the vehicle has reached operating conditions, may be ineffective to quickly respond to operating temperature variations to reduce NO.sub.x.